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<H3>bond_style fene/expand command 
</H3>
<H3>bond_style fene/expand/omp command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>bond_style fene/expand 
</PRE>
<P><B>Examples:</B>
</P>
<PRE>bond_style fene/expand
bond_coeff 1 30.0 1.5 1.0 1.0 0.5 
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>fene/expand</I> bond style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/bond_fene_expand.jpg">
</CENTER>
<P>to define a finite extensible nonlinear elastic (FENE) potential
<A HREF = "#Kremer">(Kremer)</A>, used for bead-spring polymer models.  The first
term is attractive, the 2nd Lennard-Jones term is repulsive.
</P>
<P>The <I>fene/expand</I> bond style is similar to <I>fene</I> except that an extra
shift factor of delta (positive or negative) is added to <I>r</I> to
effectively change the bead size of the bonded atoms.  The first term
now extends to R0 + delta and the 2nd term is cutoff at 2^(1/6) sigma
+ delta.
</P>
<P>The following coefficients must be defined for each bond type via the
<A HREF = "bond_coeff.html">bond_coeff</A> command as in the example above, or in
the data file or restart files read by the <A HREF = "read_data.html">read_data</A>
or <A HREF = "read_restart.html">read_restart</A> commands:
</P>
<UL><LI>K (energy/distance^2)
<LI>R0 (distance)
<LI>epsilon (energy)
<LI>sigma (distance)
<LI>delta (distance) 
</UL>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
the same as the corresponding style without the suffix.  They have
been optimized to run faster, depending on your available hardware, as
discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
manual.  The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.
</P>
<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
packages, respectively.  They are only enabled if LAMMPS was built with
those packages.  See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
section for more info.
</P>
<P>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
switch</A> when you invoke LAMMPS, or you can
use the <A HREF = "suffix.html">suffix</A> command in your input script.
</P>
<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
more instructions on how to use the accelerated styles effectively.
</P>
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<P><B>Restrictions:</B>
</P>
<P>This bond style can only be used if LAMMPS was built with the
MOLECULAR package (which it is by default).  See the <A HREF = "Section_start.html#start_3">Making
LAMMPS</A> section for more info on packages.
</P>
<P>You typically should specify <A HREF = "special_bonds.html"">special_bonds fene</A>
or <A HREF = "special_bonds.html">special_bonds lj/coul 0 1 1</A> to use this bond
style.  LAMMPS will issue a warning it that's not the case.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "bond_coeff.html">bond_coeff</A>, <A HREF = "delete_bonds.html">delete_bonds</A>
</P>
<P><B>Default:</B> none
</P>
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<A NAME = "Kremer"></A>

<P><B>(Kremer)</B> Kremer, Grest, J Chem Phys, 92, 5057 (1990).
</P>
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